Japan questioning safety of U.S. beefJUL. 15 7:50 A.M. ET Japanese food safety regulators were questioning the safety of U.S. beef after a Ministry of Agriculture study showed nearly half of the 20 mad cow cases found in Japan would have passed unnoticed under U.S. testing methods, officials said Friday.

Scientists on a Food Safety Commission panel have called for more details on a second case of confirmed mad cow disease in the United States, a move that could delay a decision to resume American beef imports, expected in late August, officials said.

The ministry report, submitted to the panel Thursday, showed that nine of the 20 cows found to have mad cow disease in Japan would have been sent to market because they looked healthy according to U.S. testing methods.

The result prompted concern among the panel, ministry spokesman Hiroyuki Kamakawa said. He said they want to assess the extent of mad cow infections in the U.S. to calculate the risk of excluding healthy-looking cows from testing. Japan tests all cows before slaughter while the U.S. only tests those that display signs of the disease.Last month, Washington confirmed that a 12-year-old cow born in Texas had tested positive for mad cow disease. It was the country's second case of the brain-wasting disease found in cattle, but the first in a U.S.-born cow. The first case was traced to a dairy cow imported from Canada.

The Japanese panel members are also concerned because the cow initially tested negative before the infection was confirmed by a British laboratory.

The panel's assessment on the safety of resuming U.S. beef imports depends on how soon Tokyo can obtain the requested information from Washington, a Food Safety Commission official said, hinting at a possible delay in the panel's decision.

Concerned about further pressure from Washington, Agriculture Minister Yoshinobu Shimamura urged the Japanese food safety experts to "reach a good decision promptly."

Japan was the United States' largest overseas market for beef before Tokyo banned all American beef after the first U.S. case of mad cow disease surfaced in December 2003.

Washington has been pressuring Tokyo to lift its 18-month ban, and some U.S. officials have threatened sanctions unless the ban ends.

Last fall, Tokyo promised it would resume limited imports of cows younger than 21 months considered less at risk of the disease, but those plans were delayed by a dispute on testing standards used to determine the age of cattle.

Mad cow disease is also called bovine spongiform encephalopathy or BSE. Eating infected beef is thought to cause variant Creutzfeldt-Jakob disease, a fatal brain disorder that has killed more than 150 people, mostly in Britain in the 1990s.